The present invention relates to an optical signal monitoring method and apparatus in wavelength multiplexing and an optical network and, more particularly, to an optical signal monitoring apparatus which performs monitoring operation with excellent feature expandability independently of the transmission rate and format of a main optical signal with an inexpensive apparatus arrangement.
Conventionally, for example, a network management/operation information acquisition means has used a method of monitoring signal quality by performing a parity check by adding a standardized overhead information bit into a transmission frame in a new synchronous network and measuring a code error rate.
A wavelength multiplexing technique as a large-volume information transmission technique can accommodate different transmission rates and formats on a wavelength basis, and can perform flexible information multiplexing.
In consideration of network management/operation, however, the use of a method dependent on the current transmission formats requires different network management/operation methods for different transmission formats. This leads to complexity of network management/operation and an increase in cost.
Demands therefore arise for techniques of acquiring network management/operation information independently of the transmission format. For example, such techniques include:
(1) a method of adding a standardized overhead information bit into a transmission frame as in a new synchronous network (see “Photonic Transport Network Architecture Employing Optical Path Concept”, Kimio Oguchi and et al., ICC' 97 workshop, Jun. 8-12 (1997));
(2) a method of obtaining a Q value from the intensity distribution histogram of the marks/spaces of a received signal (see Japanese Patent Laid-Open No. 11-223575);
(3) a method of monitoring the quality of a main signal by subcarrier-multiplexing a monitor channel with the main signal and monitoring the code error rate of the monitor channel (see “Control modulation technique for client independent optical performance monitoring and transport of channel overhead”, TuE2, OFC (2002)); and
(4) a method of monitoring code errors by setting a plurality of identification points with respect to a received signal and computing the exclusive OR of the identification results (see Japanese Patent Laid-Open No. 2000-004260).
The above conventional techniques, however, have the following problems.
In method (1), the apparatus cost increases, and the utilization efficiency of a transmission path decreases.
The reason for this is that an optical signal is monitored by defining a new frame format and setting a monitor bit in its frame overhead. Consider, for example, an optical signal having wavelengths 1, 2, 3, and 4 multiplexed. Assume that an OC-192 signal in a synchronous network is accommodated in wavelength 1; a Gigabit Ethernet (trademark) (to be referred to as GbE hereinafter) signal, in wavelength 2; a Fiber Channel (to be referred to as FC) signal, in wavelength 3; and an ATM (Asynchronous Transfer Mode) signal, in wavelength 4. In this case, the formats of the OC-192, GbE, FC, and ATM signals accommodated in wavelengths 1 to 4 must be converted into the signal formats defined in method (1). In this case, a total of four types of format conversions are required. Therefore, a device having a format conversion function must be prepared for a transmission/reception device which handles such a plurality of formats, resulting in an increase in apparatus cost. In addition, since monitor and management frame overheads are added in format conversions, the utilization efficiency of a transmission path decreases as compared with a case wherein no format conversion is performed.
When a quality deterioration occurs in methods (2) and (4), a deterioration factor cannot be specified.
The reason for this is that these methods are configured to monitor only a bit code error rate (the probability or the number of times that a mark is mistaken for a space), i.e., the probability or the number of times that a space is mistaken for a mark. In addition, quality deterioration factors of an optical signal include, for example, light noise added by an optical amplifier and the wavelength dispersion characteristics of an optical fiber. These factors cause code errors at the time of reception of an optical signal. If such a signal deterioration factor can be specified, the transmission characteristics can be improved by taking appropriate measures. If, for example, it is known that a quality deterioration has been caused by wavelength dispersion, the code error rate at the time of reception of an optical signal can be reduced by installing a wavelength dispersion compensator (Japanese Patent Laid-Open No. 11-68657) on the input stage of an optical signal receiver.
In method (3), the transmission performance of a main signal deteriorates, and the apparatus cost increases.
The reason for this is that when a monitor signal is superimposed on a main signal, the superimposed monitor signal affects the main signal to result in a deterioration in the transmission performance of the main signal. In addition, this method requires a device for demultiplexing/multiplexing a monitor signal and main signal in addition to a monitoring apparatus. This leads to increases in apparatus cost and size.